On July 13, the California Secretary of State completed the official vote count for the June 5 primary. A press release from the Secretary of State says that 31.1% of the registered voters participated. This is the lowest presidential primary turnout in California history. California has been holding presidential primaries since 1912. One can view the press release at the web page of the Secretary of State.
The Secretary of State’s press release notes that the June 2008 primary had a lower turnout, but that primary was not a presidential primary. No statewide offices were on the ballot in that primary; just primaries for Congress and state office.